1999 Dodge Truck 4 wheel drive

First check to see when you puyt into four wheel does the front drive shaft lock into four wheel to do this safely jack up driver front wheel place on jack stand in two wheel drive you sould be able to spin front shaft in four wheel you should not if this is as should be then On the front dif inside the pas wheel back side of axle tube there is a vacume accutator that locks the front into 4 wheel drive check to see if the vac. Line has come off if not. Get some help and be very careful put transfer case into 4 wheel either hi or low and with engine running in park check for vacume at plug there should be vac on the driver side of the valve. In four wheel and on the pas side of valve in two wheel if vac is there the accuator is most likely bad if no vac. There is a valve assembly that is on the transfer case that the other end of the vac. Lines go to if the line are on make sure you have the vacume supply from engine if all is good there then the switch on the transfer case could be bad can remove just unplug vac line and unscew switch there is a ball on the bottom that rides on lever inside case make sure it is free they will stick if not used. You may be able to free up if thats the case or replace the switch